With an increase in capacity of a hard disk device of recent years, a number of fluid bearing devices, which are more excellent in rotational accuracy than a ball bearing and excellent in noiselessness, are being used as bearing devices for spindle motors of hard disk devices and the like, in place of ball bearing devices conventionally used.
In this type of conventional fluid bearing device, as schematically shown in FIG. 6 and FIG. 7, a hub 52 to which a magnetic disk is fixed is mounted to a sleeve 51 having an insertion hole in a center part, and a shaft 54 driven to rotate by a spindle motor part 53 is inserted into the sleeve 51 via a predetermined clearance. A lubricant 55 is filled in the clearance between the shaft 54 and the sleeve 51. A radial bearing portion 56 comprises a dynamic pressure generating groove in a herringbone shape or the like which is formed on at least one of surfaces opposing to each other of an outer peripheral surface of the shaft 54 and an inner peripheral surface of the sleeve 51, and the lubricant 55 is also filled in this radial bearing portion 56. When the shaft 54 is driven to rotate by the spindle motor part 53, pressure is applied to the lubricant 55 due to oil feeding action of the dynamic pressure generating groove of the radial bearing portion 56, so that the shaft 54 is rotatably supported with the sleeve 51 in the posture having a predetermined amount of clearance.
In a location of the outer peripheral portion in the shaft 54, which faces an open end 57, a seal surface portion 58 is formed by being notched toward the side of a shaft axis X to have a larger clearance than the part of the radial bearing portion 56, and the lubricant 55 is also stored in the clearance between the seal surface portion 58 and the sleeve 51 even during the bearing device is being rotated. Since a relatively large amount of lubricant 55 can be stored in the location provided with this seal surface portion 58, even when the amount of lubricant 55 reduces as a part of the lubricant 55 evaporates or the like, the lubricant 55 stored in the seal surface portion 58 flows into the radial bearing portion 56 by capillary action, and the radial bearing portion 56 is always kept in the state filled with the lubricant 55 so that bearing performance is kept favorable.
Hard disk devices including those using a spindle motor or the like having this kind of fluid bearing device have been desired to be reduced in size so that they can be accommodated in smaller spaces. This requires reduction in sizes of the spindle motor and therefore the fluid bearing device.
However, in the structure in which the seal surface portion 58 is formed by notching the outer peripheral portion of the shaft 54 toward the shaft axis X as shown in the above-described FIG. 7, the sectional area of the surface of the shaft 54 itself, which is orthogonal to the shaft axis X, is small, and therefore the holding capacity of the lubricant 55 in the seal surface portion 58 formed by notching the shaft 54 cannot be sufficiently large. As a result, when the diameter of the shaft 54 is small, such a phenomenon becomes more remarkable, and a sufficient amount of the lubricant 55 may not be held.
When the outer peripheral portion of the shaft 54 is largely notched in order to increase the lubricant-holding capacity in the seal surface portion 58, the substantial shaft diameter of the shaft 54 in this location becomes extremely small, thus causing the disadvantage of reducing the shaft strength for supporting the hub 52 and the like.
In order to overcome such problem, there exists the fluid bearing device in which a seal surface portion 61 is not formed at the shaft 54, but is formed at an inner peripheral portion facing the open end 57 in the sleeve 51 as shown in FIG. 8, and this kind of fluid bearing device is disclosed in, for example, Japanese Patent No. 2937833 and so on. In this case, the seal surface portion 61 has its sectional shape in which a clearance from the shaft 54 is made by only one inclined surface which widens toward the open end 57. According to this, the seal surface portion 61 is formed at the inner peripheral portion of the sleeve 51 which is larger in diameter than the outer peripheral portion of the shaft 54, and therefore as compared with the case in which the seal surface portion 61 is formed at the outer peripheral portion of the shaft 54, the seal surface portion 61 capable of storing a larger amount of lubricant 55 can be provided. Since the shaft 54 needs not be notched, the shaft 54 can keep its diameter so that the shaft strength for supporting the hub 52 or the like is not reduced.
However, when the structure of the conventional fluid bearing device as shown in FIG. 8 is adopted, a small inclination angle θ1 of the inclined surface forming the seal surface portion 61 with respect to the shaft axis X (in FIG. 8, the intersecting portion of the inclined surface forming the seal surface portion 61 and the shaft axis X is outside the drawing, and therefore depicted instead is the equivalent inclination angle θ1 with respect to the axial line which is parallel to the shaft axis X) requires a relatively large dimension L1 as the length in the axial direction of the seal surface portion 61, in order to hold a large amount of lubricant 55. And therefore, the dimension allowed for the radial bearing portion 56 becomes small correspondingly, thus reducing the bearing rigidity. Especially, when the length of the shaft 54 itself is small as a result of reduction in size, it is difficult to form the seal surface portion 61 which can hold a sufficient amount of lubricant.
On the other hand, if an inclination angle θ2 of the inclined surface forming the seal surface portion 61 with respect to the shaft axis X is made large, as shown in FIG. 9, a large amount of lubricant 55 can be held even if a dimension L2 in the axial direction that is allowed to form the seal surface portion 61 is small, but when the shaft 54 and the like of the fluid bearing device are driven to rotate, the lubricant 55 concomitantly generates a circling flow and easily scatters outside from the seal surface portion 61 by centrifugal force. If the lubricant 55 leaks outside, there may be a threat that the lubricant 55 in the radial bearing portion 56 becomes insufficient or the hub and the like become contaminated.